Find and insert matching rows

[#1]

I have two columns, ColumnA and ColumnB. Is it possible to put their matching value with Column B? Column B values are from this list (here's the snippet code)

fileNames = Files In Directory( directory );
newlist = {};
For( i = 1, i <= N Items( fileNames ), i++, 
	Insert Into( newlist, fileNames[i] )
);
dt << New Column( "COlumn B", Character, Values( newlist ) );

Column A	Column B
Apple.doc
Orange.doc	Orange.doc
Grapes.doc	Grapes.doc
Strawberry.doc
Mango.doc	Mango.doc

[#2]

Create your Column B in a separate data table  and then use

     Tables=>Join

to merge the 2 tables together, matching Column A from  data table 1 with Column B from data table 2.

[#3 · Accepted solution]

In addition to Join Tables from Jim this would be another solution:

Names Default To Here( 1 );

// Prepare Table with Column A
dt = New Table( "Column A",
Add Rows( 5 ),
New Column( "Column A", Character, "Nominal", Set Values( {"Apple.doc", "Orange.doc", "Grapes.doc", "Strawberry.doc", "Mango.doc"} ) )
);

// Get the list
newlist = {"Orange.doc", "Grapes.doc", "Mango.doc"};

dt << New Column( "Column B", Character );

// Check each row if value is part of the list
For Each Row( dt, If( Contains( newlist, :"Column A"n ), :"Column B"n = :"Column A"n ) );

edited just to make it more clear

[#4]

One more option, quite close to the one  @Georg provided

Names Default To Here(1);

dt = New Table("Untitled",
	Add Rows(5),
	New Column("Column A",
		Character,
		"Nominal",
		Set Values({"Apple.doc", "Orange.doc", "Grapes.doc", "Strawberry.doc", "Mango.doc"})
	)
);

newlist = {"Orange.doc", "Grapes.doc", "Mango.doc"};

dt << New Column("Column B", Character, Nomimal,
	<< Set Each Value(
		If(Contains(newlist, :"Column A"n),
			:"Column A"n
		,
			""
		)
	)
);

[#5]

Thank you all for your response. It works well. 
I just want to know what does n means in 

:"Column A"n

[#6]

It is something that has replaced Name() (in JMP16 I think). It is needed if there are some specific special characters for example in column names. Here is one help article which mentions it (there is better one, but couldn't quickly find it) Scripting Guide > JSL Building Blocks > JSL Syntax Rules > Names .

I used it because I don't trust spaces in my column names even though they should be ok in JMP.